1. Field of the Invention
This invention relates to apparatus and methods for storing Liquid Natural Gas (LNG) and delivering it as vapor on demand for combustion in a vehicle engine. More particularly, the present invention relates to apparatus and methods for initializing and maintaining the pressure within an LNG storage and supply system for a vehicle powered by natural gas. While not necessarily limited thereto, the present invention is especially useful for automobiles, buses, trucks and other vehicles designed to operate through combustion of natural gas.
2. Description of the Prior Art
Various devices and methods exist for storing LNG, converting it to vapor, and delivering it at controlled pressures. Environmental and conservation demands have made it especially attractive to apply such systems for the combustion power source in motor vehicles. An example is described in U.S. Pat. No. 5,127,230 by Neeser et al which discloses a system for an LNG powered vehicle using a pair of storage tanks for the fuel. The system selects one or the other of the tanks, but does not couple them in parallel. A pressure regulation system senses the pressure within the tank, and passes the fluid from the tank through a heat exchanger to produce vapor before reintroducing it to the tank when the pressure falls too low. It is not capable of maintaining the selected tank pressure in the presence of a heavy demand for combustion gas by the vehicle. Thus, it is subject to stalling under extremely undesirable (if not catastrophic) circumstances.
Another approach, shown in U.S. Pat. No. 4,774,909 by Dolderer, is to respond to low pressure in an active tank of a motor vehicle powering system by transferring liquefied gas fuel from a holding tank. U.S. Pat. No. 4,524,747 by Wildenberg et al also employs liquid gas pumping to regulate pressure but in this case regulation is realized by injecting some liquid gas into the vaporized fuel when the pressure is too high.
Yet another variation of an LNG powered vehicle system using the direct feedback pressure regulation system is disclosed in U.S. Pat. No. 5,081,977 by Swenson which uses a bellows within the storage tank to control an output valve so as to pass liquid gas through an uninsulated pipe functioning as a heat exchanger before reintroducing it to the upper chamber of the tank.
An LNG fueling station, as described in U.S. Pat. No. 5,121,609 by Cleslukowski, transfers LNG from a storage tank to a delivery tank which is pressure regulated via a closed loop feedback system using a heat exchanger to convert LNG to vapor before introducing it to a delivery tank. A heat exchanger vaporizes the fuel to raise pressure, and liquid nitrogen cools the head gas when the pressure is too high.
U.S. Pat. No. 3,565,201 by Petsinger uses a regulator valve to allow gas to escape into an outlet line when excessive pressure is encountered in a storage tank. U.S. Pat. No. 4,995,367 by Yamauchi discloses computer controlled mixing of methanol and gasoline in a controlled ratio. U.S. Pat. No. 4,495,900 by Stockmeyer et al discloses a tank containing Zeolite fillers and parallel compartments which are sequentially switched to deliver methane to an accumulator for buffering the fuel to the engine.
The need remains in the art for a more effective fuel delivery system which is not dependent on gravity flow from the tank or tanks. The prior art LNG vehicle systems suffer from a loss of system pressure resulting in power loss or even vehicle stalling, especially in the presence of heavy demand. The problem of how to control the pressure of the fuel delivered to the engine is seriously aggravated in the prior art systems in high altitude environments. Furthermore, larger vehicles, such as buses and trucks powered by prior art LNG type systems, are even more vulnerable to the power loss and stalling problem even when the vehicle is handling a normal load demand. The prior art systems further did not effectively optimize the operating tank pressure in an LNG powered vehicle upon initial loading thereof.